This invention relates generally to a miniature motor used in power tools, for example, or more particularly to a miniature motor where the accuracy of positioning the circumferential relative positions of the core constituting the armature iron core and the commutator and the axial length accuracy of the rotor can be improved, and assembly operation is made easy.
FIG. 1 is a cross-sectional front view illustrating an example of the conventional type of miniature motor. In FIG. 1, numeral 1 refers to a case made of a metallic material, such as mild steel, formed into a bottomed hollow tubular shape, and having an arc-segment-shaped permanent magnet 2, for example, fixedly fitted to the inner circumferential surface thereof. Numeral 3 refers to an end cap made of a thermoplastic resin material, for example, and formed in such a manner as to fit to an open end of the case 1. Numeral 4 refers to a rotor comprising an armature iron core 5 facing the permanent magnet 2, and a commutator 6, and rotatably supported by bearings 7 and 8 each provided on the case 1 and the end cap 3, respectively. Numeral 9 refers to a shaft passing through the commutator 4.
Numeral 10 refers to a brush arm made of an electrically conductive material, formed into a strip shape, having at the free end thereof a brush 11 for making sliding contact with the outer circumferential surface of the commutator 6, and provided in the end cap 3. In the end cap 3 fixedly fitted with a set screw 13, for example, are a plurality of terminals 12 electrically connected to the brush arm 11. Electric power can be fed from an external power supply to windings of the armature iron core 5 via the terminal 12, the brush arm 10, the brush 11 and the commutator 6.
With the aforementioned construction, when electric current is fed to the windings of the armature iron core 5, rotating force is imparted to the rotor 4 placed in a magnetic field formed by the permanent magnet 2 fixedly fitted to the inner circumferential surface of the case 1, thereby causing the rotor 4 to rotate to drive a rotating power tool connected to the miniature motor via the shaft 9 integrally provided with the rotor 4.
FIG. 2 is an enlarged front view illustrating the vicinity of the commutator 6 in FIG. 1. FIGS. 3A and 3B are cross-sectional views taken along lines A--A and B--B in FIG. 2. Like parts are indicated by like numerals in FIG. 1 throughout. In FIGS. 2, and 3A and 3B, numeral 14 refers to a core formed by laminating thin iron sheets, for example, on which a winding (not shown) is wound to form an armature iron core. Numeral 15 refers to a boss part, 16 to an arm, 17 to a pole piece; all being integrally formed with each other. Numeral 18 refers to a groove provided on the boss part 15 at a the same circumferential relative location as that of the arm 16. In FIGS. 2, 3A and 3B, five pieces each of the arm 16, the pole piece 17 and the groove 18 are provided at equal spacings in the circumferential direction.
Numeral 19 refers to a core member made of an insulating material and formed into a hollow cylindrical shape. A plurality of commutator segments 21 formed into an arc segment shape in cross section and having essentially U-shaped tongues 20 integrally provided on the side edges of the commutator segments 21 facing the core 14 are fixedly fitted to the outer circumferential surface of the core member 19 at equal spacings in the circumferential direction to form a commutator 6. Numeral 23 refers to a projection integrally provided with the core member 19 on the end face of the core member 10 on the side of the core 14 in such a manner that the projection 23 is press-fitted to any one of the grooves 18 provided on the core 14.
With the aforementioned construction, after the core 14 and the commutator 6 are fitted to the shaft 9, the relative circumferential position of the commutator 6 is positioned in such a fashion that the projection 23 engagingly matches with any one of the grooves 18 (see FIG. 3B) on the core 14 in FIG. 2, and then the projection 23 is press-fitted to the groove 18 until the end face of the core 14 comes in contact with that of the core member 19 of the commutator 6. By doing this, both members can be positioned and fixedly fitted to each other.
With the conventional type of miniature motor, the relative circumferential positions of the core 14 and the commutator 6 are maintained by press-fitting the projection 23 into the groove 18. However, since the width of the groove 18 and the projection 23 in the circumferential direction tends to have some variability even within a predetermined dimensional tolerance, the press-fitting allowance also tends to fluctuate. This makes it difficult to press-fit the projection 23 into the groove 18.
This would result in inadequate adhesion between the core 14 and the commutator 6, and failure of assembly in extreme cases. Furthermore, the side surface of the projection 23 may be chipped off by the side edge of the groove 18, or the projection 23 may be broken during the aforementioned press-fitting operation. The resulting entry of chips into the motor components, or deteriorated positioning accuracy could lower motor performance.
By providing a clearance between the groove 18 and the projection 23, the assembly operation can be facilitated, and the chipping of part of the projection 23 or the breaking of projection 23 can be prevented. This, however, causes a phase shift between the core 14 and the commutator 6, leading to variability in the performance and life of miniature motors.
The axial length L of the rotor 4 between the bearings 7 and 8 shown in FIG. 1 is determined by the sum of the axial dimensions of a washer 24 for adjusting the protruded dimensions of the shaft 9, a thrust-receiving bushing 25, and the core 14, the commutator 6 and an adjusting washer 26 constituting the armature iron core 5, and the errors of these dimensions tend to be further aggravated. To cope with this, troublesome operations are required to prepare several types of adjusting washers 26 having different thicknesses, and use them in varied combinations to adjust the length L.